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The present invention relates generally to performance monitoring of optical networks and specifically to high resolution optical performance monitoring in wavelength division multiplexed systems.
Wavelength Division Multiplexing (WDM) is a technique for using an optical fiber to carry many separate and independent optical channels. Each channel is carried by a wavelength of light. The wavelengths between channels do not carry information but usually carry some noise. Dense WDM (DWDM) refers to the close spacing of the wavelengths carrying the channels. Current DWDM systems carry up to 160 channels spaced as closely as 50 Ghz apart with a channel power as low as xe2x88x9230 dBm before an Erbium-Doped Fiber Amplifier (EDFA). As the volume of INTERNET and other data communications traffic has increased, DWDM systems have become more in demand because of their high data carrying capacity.
The Bit-Error-Rate (BER) of an optical channel depends on four factors, the optical power level, non-linear optical distortion, electrical noise and distortion and the Optical-Signal-to-Noise-Ratio (OSNR). OSNR and the channel power are affected by an accumulation of factors including insertion loss, polarization dependent loss, and amplifier gain of the various in-line components in the system. OSNR is one of the most important parameters determining DWDM system performance because of its dominance in determining BER. Two DWDM channels having the same optical power but different OSNR have a significant difference in BER. Consequently, OSNR is typically monitored at each receiver site in a DWDM system and the OSNR information is used to optimize performance.
An additional reason to monitor OSNR in a DWDM system is the use of Optical-Add/Drop-Multiplexors (OADM). These can inject a new signal onto an unused channel of the DWDM signal or swap a new signal for an old signal in a utilized channel. When the OADM drops a signal, it drops the noise associated with that signal, reducing the noise level of the overall multiplexed signal. In addition, the signal added may have a very different power and noise level from the signal dropped. A change in the power of a channel can degrade the OSNR of other channels and the substitute wavelength may not have the needed OSNR to carry traffic if injected into routes that do not have sufficient safety margin. Each of these difficulties can be compensated for if the OSNR characteristics are measured and used to assure that the appropriate power levels are supplied.
One difficulty in OSNR measurement in any optical system is the narrowness of the optical channel linewidth (span of wavelengths used to carry information), requiring a very high resolution filter to distinguish the channel from the noise level. Conventional Optical Performance Monitors (OPM) have limited resolution when used in current systems, and thus can yield inaccurate OSNR measurement results and suboptimum performance of the DWDM system. In a DWDM signal, there is a noise floor above the zero power level and a set of channel powers at regular wavelength intervals. The OSNR for a signal channel is the ratio between the signal channel power and the noise power as expressed in Equation 1.                               OSNR          ⁢                      xe2x80x83                    ⁢                      (            dB            )                          =                  10          ⁢                      xe2x80x83                    ⁢          log          ⁢                      xe2x80x83                    ⁢                      (                                          P                signal                                            P                noise                                      )                                              Eq        .                  xe2x80x83                ⁢        1            
If the power measurement for either the channel or noise floor is incorrect, the measured OSNR will not be correct. Conventional OPMs do not measure the noise floor with sufficient resolution to provide correct readings.
Current detector circuits cannot measure the noise floor limit causing current OPMs to fail to measure OSNR values correctly. The information component of the signal has a relatively high power level (up to 30 dB greater than the noise) while the noise floor component has a very low power level. It is very difficult to design a circuit to measure a signal with the large dynamic range (up to 60 dB) that can occur in such a DWDM signal.
Three devices have traditionally been used to perform optical power measurements: the optical spectrum analyzer (OSA), an optical grating plus a detector array analyzer and the filter analyzer. The optical spectrum analyzer is a piece of laboratory equipment, large, bulky and expensive. It accomplishes bandpass filtering or splitting of the signals using a detraction grating to separate wavelengths, and a detector which measures the power in the wavelength that the signal has been broken into. The OSA can be highly accurate if enough time is allowed for enough energy to impinge on the detector. Because of the size, cost and time needed, it is not practical to utilize OSAs in a DWDM system.
The detector array analyzer uses a bulk grating and a detector array. This device satisfies the size and cost requirements for multiple deployments in a DWDM system, but has limitations as to resolution. The filter analyzer is based on a Fabry-Perot filter to determine the wavelength to be measured by the detector. If the spacing of the detector array is narrow enough, the difference between the noise and the channel can be measured. However, because the filter is designed to span multiple channels, the optical resolution is limited. Both the bulk grating and the Fabry-Perot filter can be made small and inexpensive enough to be used in multiple locations in a DWDM system, but they can only measure OSNR to 20 to 25 dB when the DWDM channel spacing is 50 Ghz or less. This limitation results in the measurement error described above and the attendant system inefficiency.
As the channel spacing decreases with increasing system capacity, it becomes more necessary to use the OSNR measurement. The best system performance can be realized by equalizing OSNR rather than power. With a built-in optical channel monitor, OSNR can be measured in realtime in the system. For long-haul systems, the OPM facilitates balancing of the optical power to minimize the effects of fiber amplifier gain non-uniformity. In addition, as an increasing number of vendors and service providers come into the DWDM market, it is desirable to use equipment (such as transmitters, optical amplifiers, and receivers) from multiple vendors in the same DWDM system. A small an economical OPM provides a useful tool for system turn-up, operation and troubleshooting in such a mixed vendor environment. Consequently, there is a need for a small, economical high resolution optical monitor that can be utilized and mounted with circuit boards implementing a DWDM system.
A high resolution optical performance monitor measures the Optical-Signal-to-Noise-Ratio (OSNR) of an optical signal, having channel and noise components. The high resolution optical performance monitor uses a notch filter to separate signal from noise, and two detectors, one for signal and one for noise measurement. The high resolution optical performance monitor is compact and economical to produce, allowing it to be used at each receiver in an optical system. When an in-fiber Bragg grating (FBG) implements the notch filter, the filter response can be made narrow-band and shaped like the optical signal thereby increasing the accuracy of separation of signal and noise. The resolution of each of the detectors can be tailored to the expected power of the channel components, thereby increasing the resolution of the optical performance monitor. The high resolution optical performance monitor provides the resolution needed for DWDM systems with channel spacing down to 50 Ghz and smaller.
A high resolution optical performance monitor tailored for single channel operation requires relatively inexpensive components. One high resolution optical performance monitor able to handle the multiplexed signals of a DWDM system utilizes a tunable notch filter in conjunction with a bandpass filter. Alternative implementations incorporate cascaded notch filters and tunable bandpass filters. The narrow-band FBG notch filter is well matched to the tolerances needed to separate signal from noise in a DWDM system.